A general ultrasound apparatus is a non-invasive test apparatus and is used to show structural details, internal tissues, and the flow of liquids in a body. An ultrasound apparatus transmits an ultrasound signal to an object and generates an ultrasound image of the object by using a response signal reflected from the object. The ultrasound image is mainly presented as a B mode image which is generated as a function of a reflection coefficient which varies based on a difference in impedance between tissues. However, an object such as a malignant tumor, for which the variation in the reflection coefficient may be relatively small as compared to surrounding tissues, is difficult to observe in the B mode image.
In particular, it is often difficult to discern, in the B mode image, a difference in dispersion efficiency between a normal tissue and an abnormal tissue. Accordingly, methods of distinguishing a normal tissue and an abnormal tissue by obtaining an elastic feature of a medium when an external pressure is applied or not applied to the medium have been suggested.
U.S. Pat. No. 5,810,731 discloses a method of obtaining an elastic feature of an object by transmitting a focused ultrasound signal to an object to induce shear waves in the object and measuring a shear wave feature.
However, in the disclosed methods, shear waves are not induced in an area which is perpendicular to a position of the object where a user locates a probe, that is, an area where a focused ultrasound signal is propagated, because the focused ultrasound signal is transmitted to the object in a direction which is perpendicular to the probe in order to induce shear waves in a direction which is perpendicular to the direction in which the focused ultrasound signal is propagated. In this aspect, even when a user locates a probe near an object in order to measure an elastic feature of a partial area of the object, according to a method of the related art, an elastic feature of a part of the object which is located under the position where the probe is located may not be obtained.
Further, according to the methods of the related art, since the shear waves are induced in the object by using the focused ultrasound signal, there may be an increased risk due to a high sound pressure of the focused ultrasound signal.